That's a bit of tough question. I'm not sure if there is a larger reason as to why it's needed but if something was too savory I might feel like it's overwhelming. I might feel like something is missing. Beer is a good example, sometimes it's too bitter, so they balance out the bitter with other things. But it tastes very bad without the balance. A cake that's way too sweet is bad, but if you adjust the ratio it tastes much better. I think pineapple adds something to pizza and for me it works. Only because it's not overwhelming and not out of balance